Step 2: Pasteurization Process

Next step required for successful mushroom growing is, pasteurization. Pasteurization is
necessary because of two major reasons. These are,
1. Pasteurization is necessary to kill any insects, pests, nematodes and fungal and
bacterial infections that may be present in the compost
2. Pasteurization is necessary to remove the excess ammonia present in the compost.
Mushroom spawns does not grow well if ammonia concentration in the compost is
higher than 0.07 percent
Well-prepared compost is packed into wooden trays before moving them into a
specially designed pasteurization chamber. Up to eight wooden trays can be stacked together as
a single unit before placing them in the room where an optimum environment is constantly
maintained. Alternatively, the bulk system may also be used for pasteurizing compost. Here
well-prepared compost is packed in a cement-block bin with a perforated floor and there is no
cover on top of the compost bin. While packing compost, care should be taken to allow gas
exchange within the compost so that ammonia is replaced by circulating air.
There are two types of pasteurization process: high temperature pasteurization and low
temperature pasteurization. High temperature pasteurization involves the raising of the compost
temperature up to 145ºF for 6 hours. This is done by injecting steam into the pasteurization
room where the compost has been placed. After pasteurization process, the temperature is
brought down to 140°F by flushing the pasteurization room with fresh cool air. Thereafter, the
compost is allowed to cool gradually at a rate of approximately 2° to 3°F each day until all the ammonia is dissipated. During low temperature pasteurization the compost temperature is
raised up to 126ºF for 6 hours by injecting steam into the pasteurization room, after which the
air temperature is brought down to 125° to 130°F range by flushing the pasteurization room
with fresh cool air. In the following 4 to 5 days, the compost temperature may be lowered at a
rate of approximately 2° to 3°F each day until all the ammonia is dissipated. The pasteurization
process can be completed within 10 to 14 days.
At the end of the pasteurization process, the compost temperature should be maintained
in the range of 75° to 80°F. The compost should have 2.0 to 2.4 percent nitrogen content and
68 and 72 percent moisture content. Make sure that pasteurization process resulted in as
homogenous a material as possible both in terms of nutrient content and temperature. A
successful pasteurization process results in 5 to 7 lbs. of dry compost per square foot of bin or
tray surface.
In some mushroom growing systems, sterilization is recommended. However
Pasteurization is highly desirable over sterilization for successful mushroom growing.
Sterilization kills all microorganisms including the beneficial ones present in the substrate
while pasteurization does not kill all organisms. Since pasteurization does not kill all the
organisms, remaining beneficial organisms present in the substrate indirectly accelerates
mushroom formation from the spawns (spores). So the purpose of pasteurization is not to get
rid of all organisms, but to get rid of harmful ones while helping to multiply beneficial ones
that discourage diseases, consume hemicelluloses, provide nitrogen, and become food for the
mushrooms. Pasteurization is very cost-effective also as sterilization process involves
expensive high pressure equipments.
Pasteurization is the most critical step in growing mushrooms. The grower must pay
close attention to the time and temperature. For large-scale production, automated machines capable of pasteurizing, cooling, spawning and filling the growing containers are available for
purchase. Even though these machines are very costly, they provide more protection against diseases and pests, and also save a great amount of human labor.